The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine and a control method of the air-fuel ratio control apparatus. The apparatus detects the air-fuel ratio of the internal combustion engine and corrects a fuel injection amount when the fuel injection amount deviates from a fuel injection amount requested for maintaining a target air-fuel ratio.
Collected Examples Of Automobile Engineering No. 95111 (date of issue: Feb. 10, 1995 by Intellectual Property Subcommittee of Japan Automobile Manufacturers Association ) discloses an air-fuel ratio control apparatus. In this example, a maximum limit of an air-fuel ratio learning value KG is increased from +20% to +40%, and a minimum limit of the air-fuel ratio learning value is decreased from xe2x88x9220% to xe2x88x9240%, so that the absolute value of an air-fuel ratio feedback correction value FAF is less. The air-fuel ratio learning value and the air-fuel ratio feedback correction value are values for correcting a fuel injection amount for maintaining the air-fuel ratio of an internal combustion engine at a predetermined value (=a target air-fuel ratio). Increasing the maximum limit or decreasing the minimum limit prevents an emission amount in gas exhausted from the internal combustion engine from increasing when a feedback control is in an open condition. The aforementioned emission indicates for example CO, HC, NOx, or etc.
The above-mentioned control is explained in detail using FIG. 8. FIG. 8 is a flowchart showing a routine in which the air-fuel ratio learning value KG is calculated in an air-fuel control apparatus of an internal combustion engine.
The routine shown in FIG. 8 begins at S1 (S1 indicates step 1, and hereinafter the same expression as this is used.) In S1, xc2x120% is set to a maximum or minimum limit of the air-fuel ratio learning value KG. In next query step, whether a deviation of the fuel injection amount reaches a predetermined abnormal level or not is determined by a detection of a fuel system by an OBD (On Board Diagnosis). That is, whether a condition of |fafkgd| greater than 35% continues for 9 seconds or more is determined in S2. Here, |fafkgd| is a fuel injection correction value (the air-fuel ratio feedback correction value FAF+the air-fuel ratio learning value KG) and corresponds to a value which is approximately equal to a deviation of the fuel injection amount from the fuel injection amount requested for maintaining the target air-fuel ratio. On the other hand, 35% is an abnormal level. When xe2x80x9cyesxe2x80x9d is determined, the routine transitions to S3. xc2x140% is set as the maximum or minimum limit of the air-fuel ratio learning value KG in S3. The calculating routine of the air-fuel ratio learning value KG then ends. When xe2x80x9cnoxe2x80x9d is determined in S2, the control returns to SI.
According to the above-mentioned example, the emission amount before the deviation of the fuel injection amount reaches the abnormal level is greater than the emission amount after the fuel injection amount reaches the abnormal level, though the deviation of the fuel injection amount before it reaches the abnormal level is smaller than the deviation after it reaches the abnormal level. The above-mentioned contradiction is caused by increasing the maximum limit and decreasing the minimum limit of the air-fuel ratio learning value KG. The maximum and minimum limits of the air-fuel ratio learning value KG are drastically changed from xc2x120% to xc2x140% when crossing the border of the abnormal detection level, as shown in FIG. 9. When the emission amount crosses over an emission regulated valuexc3x971.5 line in FIG. 9, MIL (Malfunction Indicator Light) is turned on and the disorder condition is cautioned.
FIG. 10 shows a condition in which a deviation of the fuel injection amount crosses over xe2x88x9240%. When a fuel injection correction value (FAF+KG) reaches the abnormal level, for example xe2x88x9235% as illustrated by (1) in FIG. 10, the maximum and minimum limits of the air-fuel ratio learning value KG are respectively increased and decreased from the normal value, for example xc2x120%, to a changed value for example xc2x140% as shown by (2). The air-fuel ratio feedback correction value FAF is, then, released from the restriction of maximum and minimum limits of the air-fuel ratio feedback correction value, ex. xc2x120%. This means that the line of the FAF is not on the straight line of xe2x88x9220% after the maximum and minimum limits are changed, as shown by (3) in FIG. 10. After the deviation of the actual fuel injection amount is measured and confirmed, it is determined in the next trip as shown by (4) in FIG. 10 that the fuel system is abnormal.
When the fuel injection correction value (FAF+KG) does not reach the abnormal level xc2x135% and for example the fuel injection correction value (FAFxc2x1KG) is xc2x130%, an uncorrectable deviation 10% (=30%xe2x88x9220%) which can not be corrected by the air-fuel ratio learning value KG is corrected by the air-fuel ratio feedback correction value FAF. This condition is shown in FIG. 11. FIG. 11 shows a condition where the fuel injection correction value is xe2x88x9230%. In this case, a problem, namely an amount of the emission (CO, HC, NOx, or etc.) from the internal combustion engine is more, occurs, because a deviation 10% (=30%xe2x88x9220%) can not be corrected by the air-fuel ratio learning value KG and an injection mixed by air and fuel is too rich (indicates that the fuel amount is more than required fuel amount) in a low coolant temperature or after the fuel injection is cut in decelerating driving. In this condition a feedback control is open, and the air-fuel ratio feedback correction value FAF is xc2x10%. That is, when the fuel injection correction value is less than or close to the abnormal level, the amount of emission such as CO, HC, NOx, or etc. is more than the emission amount which is exhausted when the fuel injection correction value is over the abnormal level.
It is thus one object of the present invention to solve the aforementioned problems. That is, the object of the invention is to provide an air-fuel ratio control apparatus for an internal combustion engine which in steps increases a maximum limit or decreases a minimum limit of an air-fuel ratio learning value in response to a deviation of a fuel injection amount from a fuel injection amount requested for maintaining a target air-fuel ratio. Another object of the invention is to provide a control method of the above-mentioned air-fuel control apparatus.
An apparatus for controlling an air-fuel ratio in an internal combustion engine comprises an oxygen sensor, a first operational means, a second operational means, a maximum and minimum setting means, an air-fuel ratio correcting means, a maximum and minimum magnifying means, and a maximum and minimum returning means. The oxygen sensor is in an exhaust passage of the internal combustion engine and detects a concentration of oxygen in exhaust gas from the internal combustion engine. The first operational means calculates an air-fuel ratio feedback correction value based on a value outputted by the oxygen sensor so that an actual air-fuel ratio in the internal combustion engine is equal to a target air-fuel ratio. The second operational means calculates an air-fuel ratio learning value so that the air-fuel ratio feedback correction value is within a predetermined range. The air-fuel ratio learning value is different from the air-fuel ratio feedback correction value. The maximum and minimum setting means sets a maximum limit and a minimum limit of the air-fuel ratio learning value. The air-fuel ratio correcting means corrects a fuel injection time of a fuel injection valve based on the air-fuel ratio feedback correction value calculated by the first operational means and the air-fuel ratio learning value calculated by the second operational means. The maximum and minimum magnifying means increases the maximum limit or decreases the minimum limit of the air-fuel ratio learning value by stepping degrees in response to a deviation of fuel injection amount from a fuel injection amount requested for maintaining the target air-fuel ratio after the air-fuel ratio learning value reaches the maximum or minimum limit. In this operation, the above description includes the step of increasing the maximum limit and decreasing the minimum limit or exclusively increasing the maximum or exclusively decreasing the minimum. Incidentally, the fuel injection amount is injected by the fuel injection valve deposited in an intake passage of the internal combustion engine. The maximum and minimum returning means returns the maximum or minimum limit of the air-fuel ratio learning value changed by the maximum and minimum magnifying means to a predetermined basic maximum or minimum limit of the air-fuel ratio learning value.
Since the maximum limit of the air-fuel ratio learning value is increased or the minimum limit of the air-fuel ratio learning value is decreased in response to the deviation of fuel injection amount from the fuel injection amount requested for maintaining the target air-fuel ratio, the air-fuel feedback correction value can be very small or near 0. Consequently, an emission (for example, CO, HC, NOX, or etc.) amount can be restrained smoothly without any irregular point. Furthermore, since the maximum or minimum limit of the air-fuel ratio learning value changed by the maximum and minimum magnifying means is returned to the predetermined basic maximum or minimum limit of the air-fuel ratio learning value, temporary increasing or decreasing the limit of the air-fuel ratio learning value due to a temporary occurrence of the deviation of the fuel injection amount can be restrained. Consequently, a rough control of the air-fuel ratio can be avoided.